The present invention relates to water-tube boilers such as once-through boilers, natural circulation water-tube boilers, and forced circulation water-tube boilers, as well as to burners used in these boilers.
The water-tube boiler has a body made up by water tubes. For example, the water tubes are arranged into an annular shape. In the water-tube boiler of this form, a cylindrical space defined by the annular water tube array is used as a combustion chamber. In such a water-tube boiler, primarily radiant heat transfer occurs within the combustion chamber, and then convective heat transfer occurs downstream of the combustion chamber.
In recent years, a reduction of NO.sub.x and CO emission from such water-tube boilers has been sought. The reduction in NO.sub.x, conventionally, is implemented by fitting low-NO.sub.x burners or exhaust-gas re-circulation equipment in existing boiler bodies. The reduction in CO is implemented by adjusting the state of combustion of the burner.